Polyurethane foam is prepared by the reaction of a di- or polyisocyanate with an isocyanate reactive component in the presence of a physical or chemical blowing agent. In general, one or more surfactants are required to stabilize the foam so as to produce a uniform cell structure and prevent foam collapse. The reaction is generally catalyzed, commonly by the use of tin-based and/or amine-based catalysts. A general discussion of polyurethane foams, the ingredients useful in the preparation thereof, and suitable blowing agents, surfactants, catalysts, additives, and auxiliaries may be found in the POLYURETHANE HANDBOOK, Gunter Oertel, Ed., Hanser Publishers, Munich, Germany, .COPYRGT. 1985, and in POLYURETHANES: CHEMISTRY AND TECHNOLOGY, J. H. Sanders and K. C. Frisch, Interscience Publishers, New York, 1963.
Polyurethane flexible foams have acquired a separate status in the art. In rigid polyurethane foams, the isocyanate component is reacted with isocyanate-reactive polyols having high functionality and high hydroxyl number (low equivalent weight) to produce a stiff and rigid, and generally closed cell product. Such products are useful as insulation foams and, in the higher density ranges, as structural components. Blowing of the foams may be accomplished by water as a reactive blowing agent, reacting with excess isocyanate to create carbon dioxide and provide urea linkages in addition to urethane linkages, but is more often accomplished by use of volatile physical blowing agents such as R-22, HCFC-123, HCFC-141b, pentane, or other blowing agents, sometimes in conjunction with minor amounts of water. The use of chlorofluorocarbons (CFCs) has been largely discontinued due to environmental concerns.
Flexible polyurethane foams, by contrast, are soft and resilient and predominately open-celled products that are prepared from low functionality, low hydroxyl number polyols, and are generally all water-blown or blown with water and a non-CFC physical blowing agent. Flexible foams require the use of polyether polyols and polyol blends of relatively low functionality and low hydroxyl number. A discussion of the physical properties of conventional and high resiliency flexible polyurethane foams may be found in U.S. Pat. No. 4,950,694, which is herein incorporated by reference. The useful range of polyol functionality necessary to produce an acceptable flexible foam lies between about 1.8 and 3.5, with hydroxyl numbers ranging between 10 and 180, and more often in the middle range, i.e., between 20 and 80.
For these reasons, nominally trifunctional polyether polyols in the above hydroxyl ranges have been the mainstay of polyurethane flexible foam manufacture. Such polyether polyols are prepared by the oxyalkylation of trifunctional initiators such as glycerine or trimethylolpropane, particularly the former, with propylene oxide and ethylene oxide in the presence of a basic oxyalkylation catalyst. Residual minor quantities of water contained in the initiator or introduced with the catalyst are typically removed through a stripping operation prior to beginning the oxyalkylation to insure an adequate and reproducible functionality for the final polyol product. During oxyalkylation with propylene oxide, the accompanying rearrangement of the latter to allyl alcohol and its subsequent oxyalkylation introduces polyoxyalkylene monols which lower the overall functionality. See, e.g., BLOCK AND GRAFT POLYMERIZATION, Vol. 2, Ceresa, Ed., John Wiley & Sons, pp. 17-21. Average, measured functionalities in the range of 2.2 to about 2.8 are most common. To raise or lower the functionality, it is necessary to prepare coinitiator blends with higher or lower functionality initiators or to blend polyols prepared from individual initiators of different functionality. Another alternative for raising the functionality is to add an epoxy resin during the oxyalkylation process as described in U.S. Pat. No. 4,316,991.
Glycerine, however, the most commonly used initiator in polyoxyalkylene polyether polyol production, has become a relatively high cost starting material and similarly functional lower cost replacements are not available. Additionally, stripping of residual water levels and blending of initiators or polyols or adding epoxy resins to control the functionality of the polyol leads to more complicated and costly processes.
High functionality polyether polyols (rigid polyols) suitable for rigid polyurethane foams have been prepared by oxyalkylating a number of polyhydric initiators containing four or more reactive hydrogens, such as ethylene diamine, the various toluene diamines and methylenedianilines, pentaerythritol, and saccharides and disaccharides, such as sucrose, .alpha.-methylglucoside, sorbitol, and various starch-based products. Functionalities of from 4 to 8 and higher are useful in such applications.
Oxyalkylation of polyhydric, hydroxyl-functional initiators such as the saccharides, disaccharides, and their chemically modified derivatives to produce rigid polyols is rendered difficult by the fact that such initiators are predominantly solids with elevated melting points. In many cases, the melting point is above the temperatures desired for oxyalkylation with propylene oxide, the predominate alkylene oxide used to prepare polyether polyols suitable for polyurethane applications. In many cases, even when the melting point is low, only discolored products are obtained. Thus, such initiators have generally been dissolved in an unreactive organic solvent, the heel of a previously prepared polyether polyol batch, or a reactive liquid initiator such as ethylene glycol, propylene glycol, or glycerine. However, use of previously prepared polyols as solvents results in polyols containing high molecular weight, low hydroxyl number coproducts, while use of propylene glycol or other liquid hydroxyl-functional initiators produces a polyol blend containing lower functionality species. In the latter cases, the amount of glycol or triol must be severely limited, otherwise the average functionality and hydroxyl number will be lowered below the range useful for polyurethane rigid foams. U.S. Pat. No. 5,045,623 discloses a polyol having an average functionality of 4.77 and a hydroxyl number of 490 initiated with a mixture of sorbitol (functionality 6) and propylene glycol in a weight ratio of 92:8, for example.
Sorbitol itself has been used as an initiator, or as a co-initiator/cosolvent with other saccharides or disaccharides such as sucrose, to prepare high functionality, high hydroxyl number rigid polyols, as described in U.S. Pat. No. 5,306,798. The relatively low melting point of sorbitol (93.degree.-95.degree. C.) facilitates this use. However, for such use, sorbitol must be obtained in a water-free state, thus elevating product cost. Preparation of high functionality, high hydroxyl number polyols from concentrated aqueous solutions (syrups) of sucrose containing from 10 to 20 weight percent water in the presence of urea is disclosed in U.S. Pat. No. 4,820,810. A two-stage oxyalkylation is performed, wherein unreacted water is stripped following the first oxyalkylation to leave a water content of from 8-10 weight percent or less. Following the second oxyalkylation, further unreacted water is removed. Rigid polyols having hydroxyl numbers in the range of 550 to 750 are obtained. The products are noted to contain various reaction products of alkylene oxide and urea.
In U.S. Pat. No. 4,166,172, a process for oxyalkylating aqueous solutions of polyhydric initiators, particularly sucrose, is disclosed, wherein ammonia gas is utilized as the oxyalkylation catalyst, the ammonia being oxyalkylated in the process to form alkoxyalkanolamines which also act catalytically in a later polyurethane-forming reaction. The disclosed process produced polyether polyols having hydroxyl numbers in the 500-600 range. As is the case with the process disclosed in U.S. Pat. No. 4,820,810, the water present is stated to react poorly, and thus the product has minimal diol content, ensuring high functionality.
U.S. Pat. No. 5,272,183 discloses preparation of rigid polyurethane foams from rigid polyols prepared from initiators having average functionalities of from 4 to 8, separately, or in conjunction with other initiators such as glycerine or water. The polyols, stated as suitable for low K-factor foams blown with HCFC-123 or HCFC-141b, have hydroxyl numbers between 200 and 650.
The high functionality, high hydroxyl number rigid polyols prepared as disclosed in the above-identified references are not suitable for the preparation of flexible polyurethane foams. Moreover, the introduction of substances such as urea and ammonia into the oxyalkylation process results in complex mixtures which frequently contain catalytically active species which may interfere with the production of slabstock and other commodity flexible foams.
It would be desirable to provide polyoxyalkylene polyols having hydroxyl numbers and functionalities suitable for use as the dominate polyol in flexible polyurethane foam production without employing relatively expensive initiators such as glycerine.
It would further be desirable in the production of such polyols to reduce or eliminate the stripping of minor residual quantities of water contained in initiators or introduced into initiators through addition of oxyalkylation catalysts.
It would further be desirable to produce polyols of widely varying functionality from a single initiator starting material rather than preparing multicomponent initiator mixes or by blending polyols prepared from initiators of varying functionality.
It would further be desirable to offer to the polyurethane flexible foam industry, polyoxyalkylene polyether polyols of functionality and hydroxyl number which can serve as economical replacements for conventional trifunctional polyether polyols.
It would be further desirable to provide polyoxyalkylene polyether polyol compositions which are capable of providing polyurethane flexible foams over wider processing and formulating ranges and with enhanced physical properties.